You said you adjusted the crossovers that came with the tweeters to -6dB, right? What channels are your tweeters wired into and are there other speakers wired into the same channels? Are you using two channels, or are they both going into one?
The crossovers that came with the tweeters should be your best match, so I don't think your issue is the crossover - the wrong one would allow certain frequencies to pass that the tweeter can't handle, so it wouldn't really influence volume level, more just the sound quality. I would think you either have too many speakers wired into one channel, which would pull more power than previously, or your gains are set too high.